Two types of engines currently available to power aircraft are the turbofan and the turboprop engines. Common to both engines is a power generating unit. This unit typically includes a compressor section, a combustor, and a turbine section in serial flow relationship. Pressurized air from the compressor section is mixed with fuel and burned in the combustor to produce a high velocity gas stream which expands through the turbine where energy is extracted. Some of this energy is used to power the compressor with the remainder powering the fan or propeller.
Although temperature increases occur as a result of the work done in the compressor, the highest temperatures in the engine are those in the combustor and turbine section. Pressurized air for cooling these components is typically obtained from the compressor, fan duct, or otherwise drawn in from the atmosphere.
In most turbofan or propeller driven engines, the fan or propeller is located generally forward of the core engine. Thus, in such applications, the hub structure of the blades of such propulsors operates in a relatively low temperature environment obviating the need for hub structure cooling.
However, it is known to locate the propulsor section generally aft of the core engine in a so-called "pusher" configuration. For example, Application Ser. No. 437,923, now abandoned, K. O. Johnson, discloses novel "pusher" configurations for both turbofan and propeller driven engines. Because of the close proximity of propulsor blades to the turbine and combustor in such configurations, the blade hub structures will, at certain flight conditions, be subjected to relatively high heat loads.
The air temperatures in the hub region will vary depending upon flight conditions. For example, during periods of relatively high power demand, such as takeoff, turbine and combustor temperatures are elevated resulting in higher blade hub structure temperatures. Lightweight, cost effective materials and variable pitch blade hub structures frequently have relatively low temperature limits. Thus, cooling of this hub structure may be required during such high power take-off conditions. In contrast, temperatures stabilize at a lower level during steady state cruise operating conditions and colling may not be required. Since any cooling system will have a performance penalty associated with its use, it is of interest to activate cooling only when requird. Thus, means for automatically varying the amount of cooling air to the hub region of such blades is desired.